Re: [PATCH 3/3] uprobes: Kill uprobes_mutex[], separate alloc_uprobe() and __uprobe_register()

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Oleg Nesterov <oleg@redhat.com> [2012-11-25 23:33:50]:

> uprobe_register() and uprobe_unregister() are the only users of
> mutex_lock(uprobes_hash(inode)), and the only reason why we can't
> simply remove it is that we need to ensure that delete_uprobe() is
> not possible after alloc_uprobe() and before consumer_add().
> 
> IOW, we need to ensure that when we take uprobe->register_rwsem
> this uprobe is still valid and we didn't race with _unregister()
> which called delete_uprobe() in between.
> 
> With this patch uprobe_register() simply checks uprobe_is_active()
> and retries if it hits this very unlikely race. uprobes_mutex[] is
> no longer needed and can be removed.
> 
> There is another reason for this change, prepare_uprobe() should be
> folded into alloc_uprobe() and we do not want to hold the extra locks
> around read_mapping_page/etc.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

> ---
>  kernel/events/uprobes.c |   51 +++++++++++++---------------------------------
>  1 files changed, 15 insertions(+), 36 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 2886c82..105ac0d 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -50,29 +50,6 @@ static struct rb_root uprobes_tree = RB_ROOT;
>  static DEFINE_SPINLOCK(uprobes_treelock);	/* serialize rbtree access */
> 
>  #define UPROBES_HASH_SZ	13
> -
> -/*
> - * We need separate register/unregister and mmap/munmap lock hashes because
> - * of mmap_sem nesting.
> - *
> - * uprobe_register() needs to install probes on (potentially) all processes
> - * and thus needs to acquire multiple mmap_sems (consequtively, not
> - * concurrently), whereas uprobe_mmap() is called while holding mmap_sem
> - * for the particular process doing the mmap.
> - *
> - * uprobe_register()->register_for_each_vma() needs to drop/acquire mmap_sem
> - * because of lock order against i_mmap_mutex. This means there's a hole in
> - * the register vma iteration where a mmap() can happen.
> - *
> - * Thus uprobe_register() can race with uprobe_mmap() and we can try and
> - * install a probe where one is already installed.
> - */
> -
> -/* serialize (un)register */
> -static struct mutex uprobes_mutex[UPROBES_HASH_SZ];
> -
> -#define uprobes_hash(v)		(&uprobes_mutex[((unsigned long)(v)) % UPROBES_HASH_SZ])
> -
>  /* serialize uprobe->pending_list */
>  static struct mutex uprobes_mmap_mutex[UPROBES_HASH_SZ];
>  #define uprobes_mmap_hash(v)	(&uprobes_mmap_mutex[((unsigned long)(v)) % UPROBES_HASH_SZ])
> @@ -865,20 +842,26 @@ int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *
>  	if (offset > i_size_read(inode))
>  		return -EINVAL;
> 
> -	ret = -ENOMEM;
> -	mutex_lock(uprobes_hash(inode));
> + retry:
>  	uprobe = alloc_uprobe(inode, offset);
> -	if (uprobe) {
> -		down_write(&uprobe->register_rwsem);
> +	if (!uprobe)
> +		return -ENOMEM;
> +	/*
> +	 * We can race with uprobe_unregister()->delete_uprobe().
> +	 * Check uprobe_is_active() and retry if it is false.
> +	 */
> +	down_write(&uprobe->register_rwsem);
> +	ret = -EAGAIN;
> +	if (likely(uprobe_is_active(uprobe))) {
>  		ret = __uprobe_register(uprobe, uc);
>  		if (ret)
>  			__uprobe_unregister(uprobe, uc);
> -		up_write(&uprobe->register_rwsem);
>  	}
> -	mutex_unlock(uprobes_hash(inode));
> -	if (uprobe)
> -		put_uprobe(uprobe);
> +	up_write(&uprobe->register_rwsem);
> +	put_uprobe(uprobe);
> 
> +	if (unlikely(ret == -EAGAIN))
> +		goto retry;
>  	return ret;
>  }
> 
> @@ -896,11 +879,9 @@ void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consume
>  	if (!uprobe)
>  		return;
> 
> -	mutex_lock(uprobes_hash(inode));
>  	down_write(&uprobe->register_rwsem);
>  	__uprobe_unregister(uprobe, uc);
>  	up_write(&uprobe->register_rwsem);
> -	mutex_unlock(uprobes_hash(inode));
>  	put_uprobe(uprobe);
>  }
> 
> @@ -1609,10 +1590,8 @@ static int __init init_uprobes(void)
>  {
>  	int i;
> 
> -	for (i = 0; i < UPROBES_HASH_SZ; i++) {
> -		mutex_init(&uprobes_mutex[i]);
> +	for (i = 0; i < UPROBES_HASH_SZ; i++)
>  		mutex_init(&uprobes_mmap_mutex[i]);
> -	}
> 
>  	if (percpu_init_rwsem(&dup_mmap_sem))
>  		return -ENOMEM;
> -- 
> 1.5.5.1
>